といっ + たらないCannot tell, describe, say or express how, Words cannot express, Too … for words, Extremely

といったらない is a casual expression used when something is at a level such that not even (A) is sufficient to describe it. It is usually translated as 'there are no words to express (A)', or 'too (A) for words'. This structure is a combination of the case-marking particle と, the conjunction particle たら 'when', or 'if', and the い-Adjective ない. It may be used in either positive or negative contexts.

といったらない, or the shortened form ったらない may appear after adjectives or nouns in their standard forms.

The ない in this grammar pattern is commonly thought to be an abbreviation of ありはしない, or ありゃしない, the conjunctive form of the う-Verb ある combined with the case marking particle は, and the negated form of する, しない. In this way, a more literal meaning can be thought of as 'if one were to say (A), it's not possible'.