Modern loanword from English 'backwater', used in contexts such as rowing, hydrology, or metaphorically for a stagnant situation. The cross-reference to バックウォーター suggests this is the primary modern usage.
See also: バックウォーター
The backwater of this river has risen due to the upstream dam.
standing with one's back to water
Archaic literal sense: a tactical position with water behind, leaving no retreat. Now mostly encountered in the set phrase 背水の陣 (はいすいのじん, 'burning one's boats; last stand').
The phrase 背水の陣 derives from this situation of having one's back to the water.