Slang abbreviation of 刑務所 (けいむしょ). Used in casual, often rough or criminal contexts, similar to English 'slammer' or 'pen'.
See also: 刑務所
あいつ、ムショから出てきたばかりだぜ。
That guy just got out of the slammer.
ムショにぶち込まれた。
I got thrown in the pen.
Katakana spelling is common for this slang abbreviation; usually written in kana.
Kanji spelling is rare; the word is almost always written in kana. The kanji form is the same as the second part of 刑務所 (prison).
Slang abbreviation of 刑務所 (けいむしょ), dropping the first character and using the on'yomi reading of the remaining two.