A rum do
An event that is disreputable or strange.
The 'rum' in 'rum do' is an adjective meaning 'odd' or 'strange', and nothing to do with the drink rum. The rum has reached us by a tortuous route. In the 16th century the adjectival meaning was almost precisely the opposite of the current sense - it then meant 'excellent' or 'great'. In A Dictionary of Slang and Unconventional English, Eric Partridge lists no fewer than 93 'rum' phrases, many using this earlier meaning; for example:
rum buffer - an excellent dog
rum chant - a good song
rum clout - a fine handkerchief
rum doxy - a handsome whore
rum kicks - breeches of gold brocade
These terms have now entirely disappeared from everyday language. What is unusual is that phrase with these phrase with the opposite meaning were all in use at the same time as the above:
rum customer - a dangerous fellow
rum do (or 'rum go') - a strange event
rum phiz - an odd face or countenance
rum gagger - a teller of tall tales
rum touch - an odd or eccentric fellow
There must be many more than those on Partridge's list as 'rum' could be used to precede almost anything, but the only ones to survive have the 'odd, perplexing' meaning; for example, 'rum do' and 'rum customer'.
Why the meaning of 'rum' altered isn't clear. It may be that many of the phrases that employed the earlier 'good' meaning applied to people that were good at lawbreaking or disreputable behaviour; for example:
rum bubber - a person skilled at stealing silver tankards from inns
rum fun - a clever swindle
rum diver - a skilled pickpocket
rum mizzler - a thief, adapt at escaping
rum padder - an upper-class highwayman
It could be, and this is speculation, that 'rum' migrated from simply 'good' and came to mean 'good at being bad'. From there it is a short distance to 'disreputable' and 'strange'.