Tad bit means a little amount of something and is synonymous with tidbit and titbit. Though it is always spelled as two words.

What is tad slang for?

1 : a small or insignificant amount or degree : bit might give him some water and a tad to eat— C. T. Walker. 2 : a small child especially : boy.

Why do Americans say tad bit?

It's originally and chiefly North American, according to the Oxford English Dictionary, and may be derived from “tadpole.” By the way, “tadpole” is interesting too. ... a boy,” the OED says. It wasn't until the 20th century that “a tad” came to mean a small amount or, used as a modifier, a little, slightly, or somewhat.