From most + -ly.

• (RP) IPA: /ˈməʊstli/
• (America) IPA: /ˈmoʊstli/

mostly (not comparable)

1. Mainly or chiefly; for the most part; usually, generally, on the whole.

   They're mostly good people, although they have made a few mistakes.

2. (obsolete) To the greatest extent; most.
   • ^{The template does not use the parameter(s): Please see for help with this warning.}
     1803 (date written), [Jane Austen], Northanger Abbey; published in Northanger Abbey: And Persuasion. […], volume (please specify |volume=I or II), London: John Murray, […], 20 December 1817 (indicated as 1818), →OCLC ↗:

     She was to be their chosen visitor, she was to be for weeks under the same roof with the person whose society she mostly prized [...]!

• (mainly or chiefly) by and large, in the main, more often than not, largely, predominantly, principally; see also Thesaurus:mostly

• French: surtout, majoritairement
• German: meistens, überwiegend, weitgehend, größtenteils
• Italian: soprattutto, per lo più
• Portuguese: sobretudo, principalmente, normalmente
• Russian: по большей части
• Spanish: principalmente, sobre todo