-tusten

From -us +‎ -ten, the initial -t- perhaps by analogy with -ttain.

-tusten (front vowel harmony variant -tysten, linguistic notation -tUsten)

1. Forms oppositive adverbs.

   ‎kasvot (“face”) + ‎-tusten → ‎kasvotusten (“face-to-face”)

   ‎selkä (“back”) + ‎-tusten → ‎selätysten (“back-to-back”)

   ‎vieri (“side”) + ‎-tusten → ‎vieretysten (“side-by-side”)

• sutten