Refers to social "pairings" (i.e., friendships, romantic/business relationships, marriages, etc.) that are of sufficient "durability" that they are "impermeable to moisture-related mishaps"; i.e., the couples' bond of amicable feelings with each other will not be adversely affected by stressful/unfortunate "H20-type" events that can happen in everyday life, such as getting caught in the rain, accidentally stepping/falling into deep puddles, etc.
Waterproof matches are all well and good in and of themselves, but another true --- and necessary --- test of a compatible union between two individuals would have to be a "trial by fire"... as Tevya's young daughters vehemently remonstrated in "Fiddler On The Roof", "playing with matches, a girl can get burned"!
by QuacksO April 20, 2019