What does the word whatchamacallit mean?

"Whatchamacallit" is a slang term used as a placeholder for a word you can't remember or don't know. It's essentially a casual way of saying "that thing" or "you know what I mean."

Here are some examples:

* "Can you pass me that whatchamacallit, the one with the blue handle?"

* "I went to the store to buy a whatchamacallit, you know, that thing you use to open jars."

It's a very informal word and should be avoided in formal writing or speech.