-습니다

Etymology
From earlier in the nineteenth century, contracted from, used since the seventeenth century. The uncontracted form is still found as, now archaic.

Ultimately formed by a merger of the Middle Korean verbal suffixes +  +  +.

The allomorph after sonorants represents the fact that the Middle Korean object honorific  had the allomorph  after sonorants, which became simply  after Middle Korean /z/ was (almost) unconditionally deleted in the sixteenth century.