Green’s Dictionary of Slang

obelisk n.

the penis.

[a.1556		ref. in H. Rawson Dict. of Invective (1991) 109: Pietro Aretino (1492–1556) [...] wrote a series of dialogues ... ‘Speak plainly and say ‘fuck,’ ‘prick,’ ‘cunt,’ and ‘ass’ if you want anyone except the scholars at the university in Rome to understand you. You with your ‘rope in the ring,’ your ‘obelisk in the Colosseum,’ . . . your ‘sword in the scabbard,’ not to mention your ‘stake,’ your ‘crozier,’ your ‘parsnip,’ your ‘little monkey,’ your ‘this,’ your ‘that,’ your . . . ‘job,’ ‘affair,’ ‘big news,’ ‘handle,’ ‘arrow,’ ‘carrot,’ ‘root,’ and all shit there is].
1980		Maledicta IV:2 (Winter) 188: The ingenuity of the common man [...] applied the name of practicallly every outstanding feature to the male organ — e.g. obelisk, column, monument, rod, pole, yard, halbert, drumstick, bilbo.