Definition of oneth:
(adj) :
(nonstandard) 'first', or other ordinal derivatives of 'one', such as hundred-and-oneth or minus-oneth
(adj) :
(nonstandard) Used at the end of algebraic expressions indicating ordinal position that end in 1, such as (k+1)ᵗʰ
(n) :
(in algebraic expressions) An ordinal value that is represented by an expression ending in 1 such as the (n + 1)th.
(n) :
(nonstandard) (in compounds with twenty-, thirty-, forty-, etc.) A fractional part of an integer ending in one