Late 16th century; earliest use found in Arthur Golding (d. 1606), translator. Apparently from regardful (although this is first attested slightly later) + -ness.
Definition of regardfulness in US English:
regardfulness
nounrɪˈɡɑːdf(ʊ)lnəs
Reverence, respect; attentiveness, watchfulness.
Origin
Late 16th century; earliest use found in Arthur Golding (d. 1606), translator. Apparently from regardful (although this is first attested slightly later) + -ness.