"rightly, properly; adequately, sufficiently; in accordance with duty or moral obligation," late 14c., duweliche, from dewe "due" (see due) + -liche (see -ly (2)).
I met the junior senator from Illinois and I was duly impressed
dull
dullard
dullsville
dully
Duluth
duly
duma
dumb
dumb-ass
dumb-bell
dumbfound