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