a certain person; Mr. So-and-so; an unnamed individual
Used to refer to someone whose name is withheld or unknown, often in formal or written contexts. Can carry a slightly distancing or discreet tone.
I obtained information on that matter from a certain person.
A certain person (whose name I won't mention) also attended the meeting.
Compound of 某 (ぼう, 'a certain') and 氏 (し, 'Mr./Ms.'). The reading ぼうし follows standard on'yomi. The term has been used in formal Japanese writing to refer to an unnamed person.