As an adjective slummier
is comparative of slummy.
As a noun slummer is
an inhabitant of a slum.
slummier
English
Adjective
(head)
(slummy)
slummy
English
Adjective
(er)
Like a slum; run-down, dirty, decrepit.
*2003 , (Alasdair Gray), ‘Miss Kincaid's Autumn’, Canongate 2012 (Every Short Story ), p. 745:
*:Cars and unemployment increased. Council houses took on a slummier look.