Grumpy, irritable, or difficult to deal with; also can describe handwriting that is cramped and hard to read.
From 'crab' in the sense of a crabby person, with -ed used as an adjective suffix. The metaphor likely comes from the crab's sideways, awkward movement and snapping claws.
We call grumpy people 'crabby' because crabs move sideways in a difficult, awkward way—metaphors from animal behavior have shaped how we describe human personality for centuries.
Complete word intelligence in one call. Free tier — 50 lookups/day.