Tom Swifties are a relatively recent development of the Wellerism, so we shall first look at what a Wellerism is. Brewer's Dictionary of Phrase and Fable defines it thus:

Sam Weller in Charles Dickens' "Pickwick Papers" (1836-7) was prone to producing punning sentences such as: 'Out with it, as the father said to the child when he swallowed a farden [farthing]'. This type of verbal play, involving a metaphorical and a punningly literal sense, soon gained popularity under the name of Wellerism, and a craze for devising such expressions rapidly sprang up on both sides of the Atlantic. A crude example familiar to children is: ['I see, said the blind carpenter, as he picked up his hammer and saw.']

A Tom Swifty is a Wellerism in which an adverb relates both properly and punningly to a sentence of reported speech. This is no doubt easier to see from some examples:

"The doctor had to remove my left ventricle," said Tom half-heartedly.
"Your Honour, you're crazy!" said Tom judgementally.

The quip takes its name from Tom Swift, a boy's adventure hero created by the prolific American writer Edward L. Stratemeyer. Under the pseudonym Victor Appleton, he published a series of books featuring the young Tom Swift. Tom Swift rarely passed a remark without a qualifying adverb as "Tom added eagerly" or "Tom said jokingly". The play on words discussed here arose as a pastiche of this, coming to be known by the term Tom Swifty.

In a true Tom Swifty, it is an adverb (word specifying the mode of action of the verb) that provides the pun, as in these examples:

"Elvis is dead," said Tom expressly.
"I swallowed some of the glass from that broken window," Tom said painfully.

But frequently the pun occurs in the verb, and there may not be an adverb at all. Strictly speaking such puns are not Tom Swifties, but they are generally included in the term.

"My garden needs another layer of mulch," Tom repeated.
"You must be my host," Tom guessed.

And sometimes it is neither a verb, nor an adverb, but a short phrase (usually acting like an adverb in modifying the verb):

"I've only enough carpet for the hall and landing," said Tom with a blank stare.
"Don't let me drown in Egypt!" pleaded Tom, deep in denial.

Traditionally Tom is the speaker, but this is by no means necessary for the pun to classify as a Tom Swifty. Sometimes the pun lies in the name, in which case it will usually not be Tom speaking:

"Who discovered radium?" asked Marie curiously.
"I'm going to end it all," Sue sighed.

Many – probably most – Tom Swifties are morphological; i.e. the words must be broken down into morphemes (smaller components) to understand the pun. This is true for many of the examples on this page, and is illustrated particularly well by these:

"This is the real male goose," said Tom producing the propaganda.
"The cat sounds as if she's happy now she's been fed," said Tom purposefully.

Often the adverb (or whatever) has a homonym (a word which is pronounced, and perhaps spelled, the same, but has a different meaning) which leads to the punning meaning of the sentence:

"I have a split personality," said Tom, being frank.
"I love hot dogs," said Tom with relish.

There is a special kind of homonym called a homophone. Homophones are homonyms which are spelled differently. These also contain homophones:

"There's no need for silence," Tom allowed.
"I won't finish in fifth place," Tom held forth.

[This description was adapted from:
www.fun-with-words.com/tom_sw...y.html]

http://tribes.tribe.net/tomswifties