From curt +‎ -ly.

curtly (comparative more curtly, superlative most curtly)

1. In a curt manner; tersely, almost rudely.
   • 1960, P[elham] G[renville] Wodehouse, chapter XVIII, in Jeeves in the Offing, London: Herbert Jenkins, →OCLC:

     She greeted me with a bright smile, and said: “Back already? Did you find it?” With a strong effort I mastered my emotion and replied curtly but civilly that the answer was in the negative. “No,” I said, “I did not find it.”