The story
Seny is Catalan for good sense: level-headed, practical judgement, the ability to weigh a situation before acting. The word goes back through Old Occitan to a Germanic root meaning sense, and dictionaries list its opposite in the same breath: rauxa, a sudden rash outburst, from Latin rabies, rage. Many Catalans regard seny as a trait of their own culture, and keep rauxa beside it as the other side of the character.
Seny has long been treated as a Catalan virtue. At the end of the 19th century Josep Torras i Bages, bishop of Vic, became interested in how it was handed down through proverbs and moral stories. He encouraged the writer Josep Abril i Virgili to collect them, and the result, illustrated by Joan Junceda, was a book called Bon seny, 'good sense'. Under Franco, when printing in Catalan was heavily restricted, copies became rare.
The word is still in daily use. Someone sensible is assenyat. A wisdom tooth is a queixal del seny, a tooth of good sense. And the motto of the castellers, the teams who build human towers at festivals, lists four values: strength, balance, courage and seny.
English says common sense, which gets about half of it. Common sense is what anyone ought to have. Seny sounds more like something earned with age, the steady head of the person who is asked what to do when everyone else is shouting.
There is also an old, unrelated seny meaning a large bell, from Latin signum.