Green’s Dictionary of Slang

agate n.

[the carving of tiny figures into SE agate, a semi-precious stone]

1. [late 16C–early 17C] a very small person.

2. [1960s+] (US) a small penis.

3. see glassy (alley), the n.

SE in slang uses

In phrases

shoot the agate (v.) [the extension of the thumb when playing marbles; note nonce sense by Trimble, in 5000 Adult Sex Words & Phrases (1966): ‘shooting the agate [...] Looking for a chance love partner by walking along with hands at the sides and thumbs extended. In hipster code, this means that a man is asking for sexual companionship, and if a hip female is interested, she may respond without a word being spoken’]

[1900s–40s] (US black) to walk jauntily with one’s thumbs extended.