'sloppiness' originates from English, specifically the adjective 'sloppy' plus the noun-forming suffix '-ness', where 'sloppy' is formed from the word 'slop' + '-y' and 'slop' referred to a wet or messy substance.

'slop' appeared in Middle English (e.g. 'sloppe') possibly from Low German/Dutch sources meaning a loose garment or watery refuse; by the 17th–18th centuries 'slop' had senses of liquid waste or mess, then the adjective 'sloppy' (late 18th–early 19th century) meant 'wet and messy' and later 'careless', and 'sloppiness' developed as the abstract noun from 'sloppy' + '-ness'.

Initially related to wetness or a messy liquid (physical mess), the sense broadened to describe general untidiness or lack of care and now commonly means 'careless, untidy, or negligent behavior or quality'.