Deuce in the Caboose

To defecate in one's drawers. Typically used to affectionately refer to a toddler pooping in their diapers. Adults see the droopy diaper from behind the toddler, or the caboose / rear-end.

Refer to related phrases: drop a deuce, deuce

Ohhhh, how cute! Your toddler is waddling away from the table...wait a minute! There appears to be a 'deuce in the caboose' of theirs!