Native speaker
In Emirati Arabic, “Triangular” is مُثلث (muthalath).
مُثلث
Triangular
It's in the phrasebook under Yes, no & everyday basics, with 90 more like it.
imdawwar
mur
'areedh
taazij
dha'eef
haamudh
Source: DLIFLC Emirati Basic Language Survival Kit (2007). Credits and licences