"Are you lost, Daddy?" I asked tenderly.
"Shut up," he explained.
— The Young Immigrants by Ring Lardner
A said-bookism is a word that is used in place of the word 'said', but probably shouldn't have been. Writers are fortunate to have been gifted with one word that they rarely have to question: the all-mighty 'said'. When your characters are engaging in dialogue, they may on occasion have reason to gush or shriek or question or elaborate, but for 90% of the conversation they can simply say what they mean to say, and be done with it. The reader is so used to seeing the word 'said' that it will not become boring with overuse or distract the reader from the action. It is a free word, one that should be used liberally.
"I'm exclaiming," he exclaimed. And that's when he knew that he was in a shitty novel.
In the days of Penny Dreadfuls and Dime Novels, it was common to try to spice up an otherwise bald and unconvincing narrative by using clever wordplay, or failing that, clever words. Or failing that, just words. The Tom Swift books became so well-known for this that the punning said-bookism has become known as the Tom Swifty. It is claimed that the term 'said bookism' comes from a mail-order booklet that aspiring writers might order, consisting of lists of vocabulary with which to empurple their prose. It is uncertain if this is true, although I am certain such lists were available in one form or another.
"Don't do these things. Please oh please. The best form of dialogue attribution is said, as in he said, she said, Bill said, Monica said."
-- On Writing by Stephen King
It is important to note that many segments of dialogue will not even need to use 'said'. It is often quite clear who is speaking, and there's no point in adding words that tell the reader nothing. If you do need dialogue markers, words like 'asked', 'told', and 'answered' may also be light-weight enough that they can be used without distracting the reader from the dialogue, and you can usually use them without charges of said bookism being leveled. On the flip side, it is perfectly fine to use descriptive words when they are called for, and some writers, particularly those of the literary sort, will turn said bookism into an art form. The works of Louis de Bernieres and Gabriel Garcia Marquez, for example, gain some of their charm from the fact that what little dialogue they have is buried beneath a mountain of elaboration.
"The old soldier wagged his head, rolled his goggle-eyes, until I expected to see them slip out of their sockets; placed his dirty forefinger by the side of his broken nose; solemnly ejaculated 'Coffee!' and immediately ran off into an inner room."
-- A Terribly Strange Bed by Wilkie Collins
Unfortunately, said bookism is still very much an Ism for some writers. Even more unfortunately, it is actively and explicitly taught -- and not just encouraged, but insisted upon -- in American public schools. This appears to be because some tests will give more points if a writing passage includes many different vocabulary words, and 'said' just doesn't win many points, especially if used repeatedly. This takes a toll in later years when these children start writing fanfic and/or romance novels.