Wow - look at this! (Merriam-Webster http://www.webster.com)

Main Entry: tit
Pronunciation: 'tit
Function: noun
Etymology: Middle English, from Old English -- more at TEAT
Date: before 12th century
1 : TEAT
2 usually vulgar : BREAST -- usually used in plural

Actually I was moderately surprised to find it listed, let alone as first meaning. Looks like Heinlein made a difference