1590s, "flattering," a sense now obsolete, present-participle adjective from soothe (v.). The sense of "mollifying" is attested by 1746. Related: Soothingly.
Entries linking to soothing
soothe v.
Middle English sothen, from Old English soðian "show to be true, bear witness, offer confirmation" (senses now obsolete), from soð "true" (see sooth). The sense of "quiet, comfort, restore to tranquility," in reference to a person or animal, is by 1690s, via the notion of "to assuage one by asserting that what he says is true," a sense attested from 1560s (and compare Old English gesoð "a parasite, flatterer"). The meaning "reduce the intensity" (of a pain, etc.) is from 1711. Related: Soothed; soother; soothing.