Left three-drops water radical — the side-form of
水, three abbreviated waves. The indexing radical, anchors
溃 in the water family with
河 river,
海 sea,
流 flow. Marks
溃 originally as a dike collapsing under floodwater — the picture behind 'to burst, to rout, to fester' (an ulcer is flesh 'breaking open' the way a levee does).