-u
From open-dictionary.com - the free dictionary.
Finnish
Deverbal suffix
-u (with back vowel harmony)
-y (with front vowel harmony)
- Used to form nouns, usually, from verbs whose citation form ends with -ea, -eä, -ia or -iä.
- hyppiä to be jumping -> hyppy jump
- itkeä to cry -> itku cry(ing)
- potkia to kick -gt; potku kick