the.com/waller

means Someone who props themselves against a wall or doorway instead of standing on their own — a professional loiterer in good posture.

from Straightforward agent-noun territory: 'wall' plus the '-er' that turns any noun or verb into 'one who does it' (baker, boxer, waller-of-walls). The wall-builder sense — a craftsman who stacks stone walls without mortar — is the older trade meaning, still used in parts of Britain; this cheekier sense just borrows the shape of the word for the person doing the leaning, not the building.