Secondly means in the second place. Firstly, secondly, and thirdly, secondly there is the cost.
It marks the second point in a list, and it belongs to speeches and essays. Some writers prefer first, second, third to firstly, secondly, thirdly, on the grounds that the shorter forms do the job — but both are correct and always have been.
1adverbsecondlyIn the second place.
“Firstly, it's too expensive, and secondly, we don't have room for it.”
secondly is a connective.
Proto-Indo-European
*sekʷ-
Middle English
secunde
English
second
English, today
secondly
Built from
From second + -ly. second comes from Middle English secunde. From Middle English secunde, second, secound, secund, borrowed from Old French second, seond, from Latin secundus (“following, next in order”), from root of sequor (“to follow”), from Proto-Indo-European sekʷ- (“to follow”). a doublet of secund and secundo. Displaced native twoth and partially displaced native other (from Old English ōþer (“other; next; second”)).
Etymology from Wiktionary, CC BY-SA