Correct spelling for the English word "hastler" is [hˈe͡ɪslə], [hˈeɪslə], [h_ˈeɪ_s_l_ə] (IPA phonetic alphabet).
Corrupted into hastener.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.