An odd and inexplicable bit of nautical slang. It first appeared in written English in 1865 in a dictionary of nautical slang (Hotten's Slang Dictionary), where it was spelled 'manablins' and defined as 'broken victuals'. It has variously been spelled manavilins, manavalums, malhavelins, manarolins, and manavelings. Sailors were not big on careful pronunciations in days of yore, and were even less keen on correct spelling.
It is variously used to mean 1. small, miscellaneous matters, 2. odds and ends, particularly of nautical gear and 3. stopgap provisions.
Early on it made a brief appearance in the world of the railway, where it was used to refer to the extra money left in the till after the day's accounts were settled, which was split up between the clerks.
It is always used in the plural, but rarely used at all. It appeared out of nowhere, failed to have anything that might be identified as a heyday, and faded slowly into obscurity. It is one of the marginalized words that we never hear of.