Literally: tense – loincloth – one – [ordinal counter]
Alternately: To brace yourself mentally for something. To prepare to face difficulty or hardship, or at least a high-level competition of some sort. Literally, to gird one’s loins.
Notes: 緊褌 refers to tightening up one’s loincloth; the 一番 refers to the seriousness of the situation, or of the person’s emotional state as they prepare. The character 褌 on its own can be read as fundoshi, a term that might be familiar to sumo enthusiasts.