probably

• probablely (obsolete)
• prolly (colloquial, slang)
• proly (leetspeak)

From Middle English probably, probabily, equivalent to probable +‎ -ly.

• (Received Pronunciation) IPA^(key): /ˈpɹɒbəbli/, (colloquial) /ˈpɹɒbli/
• (General American) IPA^(key): /ˈpɹɑbəbli/, (colloquial) /ˈpɹɑbli/, (colloquial) /ˈpɹɑəbli/

• Audio (US)

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probably (comparative more probably, superlative most probably)

1. In all likelihood.
   • 1921, Ben Travers, chapter 1, in A Cuckoo in the Nest, Garden City, N.Y.: Doubleday, Page & Company, published 1925, →OCLC:

     “ […] the awfully hearty sort of Christmas cards that people do send to other people that they don't know at all well. You know. The kind that have mottoes […] . And then, when you see [the senders], you probably find that they are the most melancholy old folk with malignant diseases. […] ”

   • 2013 May–June, William E. Conner, “An Acoustic Arms Race”, in American Scientist, volume 101, number 3, pages 206–7:

     Earless ghost swift moths become “invisible” to echolocating bats by forming mating clusters close […] above vegetation and effectively blending into the clutter of echoes that the bat receives from the leaves and stems around them. Many insects probably use this strategy, which is a close analogy to crypsis in the visible world—camouflage and other methods for blending into one’s visual background.

(in all likelihood):