ガチンコ

From がちん (gachin, “thuddingly”, adverb; sound effect for something heavy and solid colliding) +‎ こ (-ko, nominalizer). Compare ぶらんこ (buranko, “(playground) swing”), from adverb ぶらん (buran, “hanging loosely”) + こ (ko, nominalizer).

• (Tokyo) ガチンコ [gàchíńkó] (Heiban – [0])[1]
• IPA^(key): [ɡa̠t͡ɕĩŋko̞]

ガチンコ or がちんこ • (gachinko) 

1. (sumo) a real match with no allowance to one's competitor, as opposed to a thrown or an arranged match
2. a no holds barred, do-or-die, serious competition or challenge

   ガチンコ対(たい)決(けつ)

   gachinko taiketsu

   a showdown, a do-or-die confrontation

• ガチ (gachi, “serious”, adjective)

• がちがち (gachigachi)
• がちり (gachiri)
• がっちり (gatchiri)
• がちん (gachin)
• かちかち (kachikachi)
• かちり (kachiri)
• かっちり (katchiri)
• かちん (kachin)
• かちんこ (kachinko)

• Entry at Nihon Jiten (in Japanese)