In dated|lang=en terms the difference between crummy and rummy
is that
crummy is (dated) soft, like the crumb of bread; not crusty while
rummy is (dated) peculiar; odd.
As adjectives the difference between crummy and rummy
is that
crummy is (informal) bad; poor while
rummy is (dated) peculiar; odd.
As nouns the difference between crummy and rummy
is that
crummy is small van, bus, or railway car used to transport loggers or other resource workers to and from the worksite a common term when referring to a vehicle with a compartment separate from the cab, housing the silvicultural, logging or mining crew during transport while
rummy is (uncountable) a card game with many rule variants, conceptually similar to mahjong.