Some I came up with before for my conlang:
chew air – be poor
catch a fish with mere hands/teach a wall to part – try the impossible (from a sceptic viewpoint)
have eyes like sponges – be a crybaby
break someone's eyes – to delude someone
spit books – be a smartass
have bathed in knives – bleed everywhere; bleed to death
serve last week's leftovers (for the 3rd time) – bring up a topic that's been done to death