Mark your card

Does anyone know what 'Mark Your Card' means? I know someone at work who says it all the time, he seems to mean he needs to tell me something about something or someone, like giving me a 'heads up', but I'm not sure...

I've never heard this one. It might mean "Take note of this." Better ask him. ~rb

Confess the same ignorance. Mark your dance card, Mark your calendar, Mark my words! just don't do it.
SS

Bingo players also mark their cards, as do many who keep scorecards of some sort, e.g. golfers.

See also: the meaning and origin of 'Heads up'.