The story
From bedwetting (“a type of urinary incontinence involving involuntary urination while in bed during sleep”) + alarm.
Etymology from Wiktionary, CC BY-SA 4.0.
bedwetting alarm
Today it means: A small device inserted into a person's undergarments during sleep that activates an alarm when urine is detected by its sensor, intended to wake the individual so they cease bedwetting. Full definition →