Green’s Dictionary of Slang

hang v.7

[1960s+] (orig. US) to turn a corner in a motorcar; as in hang a left, hang a right.

In phrases

hang a lilly (v.) [initial letters]

[1960s+] to turn left.

hang a ralph (v.) (also hang a roscoe) [initial letters]

[1960s+] to turn right.

hang a Sam (v.) [initial letters]

[1960s] (US) to go straight on.