soil (one's) diaper(s)
To defecate into the diaper one is wearing. Primarily heard in US, Canada. Uh oh, I think Tommy just soiled his diaper. I'd better go change it. I just hope I don't live so long that I end up in some nursing home soiling my diapers.
See also: soil
soil one's diaper(s)
[for a baby] to excrete waste into its diaper. The baby soiled his diapers. I detect that someone has soiled his diaper.