ביסט
'are (you) (second person singular)'

ETYMOLOGY

I. Proto-Yiddish ✱bist; Proto-EY ✱bist; from MHG bist; cf. NHG bist.
Protovowel I1.
II. MHG bist; from OHG bist.
III. The verb 'to be' in Germanic languages is suppletive, combining forms from three different PIE verbs. The form ביסט (MHG  bist) is from PIE root ✱bheu- 'become, grow'.
Lühr argues that the final -t was adopted in OHG on the pattern of the Pretero-present verbs and that the initial b- was adopted to differentiate the resulting ✱ist from the 3rd singular ✱ist.

WESTERN

Alsace (France)

voheːʁ- bɩš וואָהער ביסט פ פּ {ALSACE, 47075 Mulhouse, 008-041}

CENTRAL

Unterland (East Transcarpathian)

bɪ́stə פֿון וואַנען ביסטו {ETCP, Nyzhnya Apsha, 48233}
- bɩsčɔjn zat? ביסט שוין זאַט

dᵻ bᵻs nᵻʃ' fᵻn danɛ דו ביסט נישט פֿון דאַנען {Warsaw, Geller 2001: 175}
fn̩ vanɩn ˈbɩstɩ פֿון וואַנען ביסטו {POLAND, Warsaw, 52211}