The Japanese word for capsicum is
唐辛子

とうがらし - tougarashi

Translations for capsicum and their definitions

ピーマン
1. n. (from 1961) bell pepper, sweet pepper
2. n. (date uncertain) (slang) someone who is stupid, vapid, daft, not very bright (from the way that a bell pepper is mostly hollow inside, as a metaphor for such a person's head being empty)

唐辛子
1. n. chilli pepper
2. n. Usage: As with many terms that name organisms, this term is often spelled in katakana, especially in biological contexts, as トウガラシ