1520s, "by the way, casually;" see incidental + -ly (2). Sense of "as a new but related point" attested by 1925.
incidentally, I won't go to the party
these magnificent achievements were only incidentally influenced by Oriental models
inchoative
inchworm
incidence
incident
incidental
incidentally
incinerate
incineration
incinerator
incipience
incipient
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