Definitions

1. noun

The ordinal number indicating a position that is twenty or less, especially a number between 11 and 19; an eleventh, twelfth, thirteenth, etc., number.

“The umteenth day of the month brought a surprise celebration.”

2. adjective

Of or relating to the ordinal number twenty or less, especially a number between 11 and 19.

“The umteenth anniversary of their wedding was a grand affair.”