single horseman; lone rider
Historical or literary term for a lone mounted warrior. In modern contexts, it can be used figuratively for someone acting alone, but the base meaning is a single horseman.
He rode alone into the enemy camp.
waiting for a single tile to complete a pair and hand; waiting for half of a pair with four melds completed
Mahjong term: a tenpai state where the player needs one specific tile to complete the pair (head) and thus the whole hand, often with four melds already formed. The cross-reference 単騎待ち (たんきまち) is a related term.
See also: 単騎待ち (たんきまち)
This hand is waiting on a single tile.
waiting for one tile to complete a pair with an otherwise exposed hand
waiting for a single tile to complete a pair and win; waiting for one tile to finish a pair with four melds already completed
From 単 (single) + 騎 (horseman, rider). The mahjong sense is a metaphorical extension, likening the lone tile needed to a single rider.