means A dating app where you swipe right to like someone and left to pass, matching only when both people swipe right; also, the older word for dry material that catches fire from a spark.

from From Old English 'tynder,' the easily-kindled stuff—dry grass, wood shavings, scorched cloth—you'd nudge a spark into to coax up a flame, related to the verb 'to tind,' meaning to kindle. It's a cousin of Old Norse 'tundr' and German 'Zunder.' The app borrowed the metaphor wholesale: a spark, something dry waiting to catch, and the optimistic hope that all of it might burst into something warm rather than just smolder and die.