Green’s Dictionary of Slang

favour n.

In phrases

do me a favour

see separate entry.

I could do her a favour (also I could do that a favour) [the favour would, of course, be sexual; thus note 18C the favour, sexual intercourse]

a remark made by a man of a passing woman.

[UK]Partridge DSUE (8th edn) 587: [...] since ca. 1930.