대여섯 (taeyeoseot) means “five or six”

More examples

컵에 대여섯 개 남아 있어요.keo be tae yeo seot kae na ma i sseo yo

There are five or six left in the cup.

A rough count at the narrow boundary between five and six.

다섯taseot: The exact number five.
여섯yeoseot: The exact number six.
서너seoneo: A rough amount of about three or four.

정확하지 않고 어림잡아 다섯이나 여섯임을 나타내는 구어적 표현.cheong hwa ka ji an ko eo rim ja ba ta seo si na yeo seo si meul na ta nae neun ku eo jeok pyo hyeon

No reliable etymology is established for this expression, which is best learned as a conventional informal estimate of five or six.

English speakers can use it like a casual estimate, not like a fixed mathematical quantity, so the surrounding situation determines the intended count.