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 ...
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