(mostly) British Slang
In a strictly dictionary sense, a twerp is a person regarded as insignificant and contemptible. The Dictionary of American Slang and Colloquial Expressions defines the it as "an annoying runt of a person.
In an everyday, idiomatic sense it means a small individual or child. The word is most frequently used in an affectionate sense, as in "You silly twerp!" when someone has done something unfortunate, like dropping a full milk bottle as a small child. For me the word conjures anew the memory of my father telling me off, on those occasions when I did drop a milk bottle or track mud in onto the carpet. In later years, I was told by my peers (who I naturally believed) that it was a term used to describe a pregnant goldfish.
So where did the word come from? Some point to a letter written by J.R.R. Tolkien to his son Christopher Tolkien on the 6th October 1944. Tolkien refers to one T.W. Earp (President of the Oxford Union before the Great War), who he calls "the original twerp". In point of fact, the word was in use in the Army as early as 1925, and one source points to a date of 1874.
My efforts to prove that it is also the oft-rumoured pregnant goldfish have so far come to naught. My peers were clearly twerps.