Mid 17th century; earliest use found in Walter Charleton (1620–1707), physician and natural philosopher. From un- + regardant, probably after unregard.
Definition of unregardant in US English:
unregardant
adjective(ˌ)ʌnrɪˈɡɑːd(ə)nt
rare, poetic
Unobservant, inattentive; dismissive.
Origin
Mid 17th century; earliest use found in Walter Charleton (1620–1707), physician and natural philosopher. From un- + regardant, probably after unregard.