England and Wales have had strong cross-linguistic contact for as long as either language has existed. As is to be expected of the English language, it has absorbed several words from Welsh. The accuracy of many such etymologies is debatable, and many of these absorbed words come from more possible languages than Welsh, or passed through intermediate languages before reaching English in their current form. English and Welsh are both heavily informed by Latin word migration, so there are many English and Welsh words which are nearly identical, but which arrived in English and Welsh independently, directly from Latin, or indirectly from French during the Norman invasion of England.
In this writeup, I am first focusing specifically on words which have definite Welsh origin, and second I am including words which are commonly cited as having definite Welsh origin, but which do not hold up well under etymological scrutiny. Categories are divided in decreasing order of our certainty of the Welsh origin of these words.
Words definitely or probably derived directly from Welsh:
Avon - place name of a specific river, Welsh afon "river"
brock - a rough-looking or odorous man, a badger, Welsh broch "badger"
coomb - a type of valley, Welsh cwm, same meaning
corgi - small dog breed, Welsh corgi, from cor "dwarf" + ci "dog"
cotton (verb) - to get along with someone, to be agreeable, Welsh cytuno "consent, agree."
flummery - a type of pudding, from Welsh llymru "sour oatmeal jelly boiled with the husks"
gull - seabird, Welsh gwylan "gull," Cornish guilan, Breton goelann
tor - prominent hill, Welsh twr "heap, pile"
Words sourced certainly from multiple simultaneous languages, including Welsh:
bard - a traveling poet and musician, Welsh bardd, Scottish bard, both from Old Celtic bardos "poet, singer." At the time of the original use of the source words, it was pejorative in Scotland, and a term of respect in Wales.
coracle - hidebound wicker boat, Welsh corwgl "small boat," Irish Gaelic curachan "boat," Middle Irish curach "boat"
crag - rocky outcropping or cliff, Welsh craig "rock, stone," Old Irish crec "rock" and carrac "cliff," Manx creg, Breton krag
glen - narrow valley, Welsh glyn, Old Irish glenn, Scots Gaelic gleann, all meaning "valley"
wrasse - species of saltwater fish, Welsh gwrach "hag, witch," Cornish wrach "wrasse fish"
Words sourced uncertainly from multiple possible languages, including Welsh:
balderdash - Welsh baldordd “idle noisy talk, chatter," or Danish balder "noise, clatter"
bug (as ghost; bugbear, bugaboo), bogey, boogie-man - Welsh bwg "ghost, goblin" (compare Welsh bwgwl "threat," earlier "fear"), Scottish bogill "goblin, bugbear," Middle Irish bocanách "supernatural being," German bögge, böggel-mann "goblin."
car, cart - Latin carrum, carrus (plural carra), originally "two-wheeled Celtic war chariot," itself from Gaulish karros, itself from Old Irish and Welsh carr "cart, wagon," Breton karr "chariot"
dad - father, Welsh tad, Irish daid, Czech, Latin, Greek tata, Sanskrit tatah, all of the same meaning
dun - dark, dusky, or dingy in colour, Old English dunn "dingy brown, dark-colored," itself from Old Irish donn "dark," Welsh dwnn "brownish"
flannel - Welsh gwlanen "woolen cloth," from gwlan "wool," alternately Old French flaine "coarse wool"
pooka, Puck - Welsh pwca "goblin, tricky spirit" and several others; c.f. bug above, for virtually identical etymology.
truant - Welsh truan "wretched, miserable," Breton truan "vagabond"
Words that English may have acquired from Welsh, or that Welsh may have acquired from English, but we aren't sure which is which due to the timing of these words' first appearances in each language, and due to a lack of clear connections to other source languages:
clutter - untidiness, a mess consisting of many objects, Welsh cludair "heap, pile"
crumpet - a variety of pastry, Welsh crempog "pancake, fritter"
hassock - clump of long grass, Welsh hassock
trousers - Welsh trwser "pants, trousers"
whop - to beat or strike, Welsh chwap "a stroke"
Very dubious folk etymology:
penguin - Welsh pen "head" + gwyn "white," thought to refer to the great auk, but this is inconsistent with compound word formation in Welsh, and there is an approximately equally adequate possible Latin etymology for this word, as well.
Words which definitely did not come from Welsh, but have been cited as coming from Welsh:
druid - Welsh obtained derwydd from Old Celtic; English obtained druid directly from Latin, by an etymology which predates the passage of the word into Welsh.
taffy (when referring to the food substance) - confused with the English perversion Taffy of the Welsh man's name Dafydd; Taffy is a slang term for a Welsh man, but taffy in reference to food comes by an unknown but possibly West Indian origin.
Other Nodes of Interest:
English words of....
English words of Celtic origin
Iron Noder 2015, 03/30