Where did the term "hard on" come from?

I haven't a clue. Anyone else?

POSSIBILITY: HARD is self explanatory. ON is a obsolete term for close. HARD ON seems to indicate an erection close by; "I have a HARD ON for her." The term HARD ON was identified as sexual slang back to about 1890, so this is certainly possible.

When it's not soft I've got a hard on ( massive firm erection )

So I take it it means something like: I've got a hard going on for xxxx ?

Sort of like 'the lights are on'?

Sounds plausible ...