Green’s Dictionary of Slang

1. sexual intercourse in which the partners are standing up.

1937–84		Partridge DSUE (1984) 871/1: mid-C.19–20.
1951		Cusack & James Come in Spinner (1960) 263: I’m not good at perpendicular love-making myself, but these coots all seem to be fitted with a gyroscope.

2. a meal taken standing up, a party at which the guests stand rather than sit in a formal ‘placement’.

1873		Sl. Dict. 251: Perpendicular a lunch taken standing-up at a tavern bar. It is usual to call it lunch, often as the perpendicular may take the place of dinner.
1895		C. Crowe Aus. Sl. Dict. 57: Perpendicular, a lunch eaten by one standing.
1953		D. Cusack Caddie viii: The typical New South Wales bar, with its ‘perpendicular’ drinking (tables would take up profitable standing room).