同士|どうしdoushi[1]atamadaka (head-high)

1. noun, noun (suffix)fellow; mutual; companion; comrade
   １０代だいの友ともだち同士どうしが徹夜てつやでおしゃべりした。↗

   The teenage friends stayed up talking all night.

• Synonym仲間なかまNeutralThe most general word for a companion or friend. 「同士」 is mostly a suffix, as in 「恋人同士」.
• Synonym同志どうしNeutralA homophone of 「同士」: a like-minded comrade, as in 「同志を募る」. 「同士」 instead marks a same-kind relationship.
• Synonym同類どうるいNeutralAn ordinary word for the same kind or category; also an accomplice or partner in crime. 「同士」, by contrast, is mostly a suffix.
• Synonym僚友りょうゆうWrittenWritten: a colleague or coworker. In everyday speech say 「同僚」.
• Synonym同輩どうはいWrittenWritten style: a person of equal status or standing. 「同士」 is mostly a suffix expressing a mutual same-kind relationship.
• Related同僚どうりょうNeutralA colleague at the same workplace. 「同士」 marks a same-kind relationship and is mostly used as a suffix.
• Related同業者どうぎょうしゃNeutralAn ordinary word for people in the same trade or line of business. 「同士」, by contrast, is mostly a suffix.
• Related同役どうやくWrittenWritten and rare: a colleague holding the same post. In everyday speech say 「同僚」.
• Related朋輩ほうばいWrittenWritten, old-fashioned: a fellow disciple under the same teacher or a workplace colleague. In daily speech say 「仲間」.
• Related輩ともがらWrittenAn archaic word for companions or people of the same kind; rarely used today.
• Related社友しゃゆうWrittenFriends or workmates at the same company; can also refer to contributing staff of a newspaper.

Use 「同士」 as a suffix for a mutual same-kind relationship, as in 「友達同士」. For companions or friends say 「仲間」; for workplace colleagues, 「同僚」; for like-minded comrades, the homophone 「同志」. 「同類」 (same kind, or accomplice) and 「同業者」 (same trade) are ordinary neutral words; 「同輩」「同役」「朋輩」「輩」「社友」「僚友」 are written, old-fashioned or rare.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 仲間なかま。同志どうし。