Green’s Dictionary of Slang

get it up v.

(orig. US)

1. [1930s+] (also get one up, get up) to achieve an erection; occas. in fig. use of a woman; .

2. [mid-19C] of a woman, to stimulate the penis to erection.

3. [1950s] (US Und.) to amass and produce a required sum of money, e.g. in payment of an outstanding debt.

4. [1970s+] in fig. use, to maintain enthusiasm for an idea, situation etc.