Pronunciation: /ˈprɒbəbli/, with primary stress on the first syllable: PROB-uh-blee. The /ɒ/ shown here is the British-style vowel you hear in words like hot; American speakers typically use a different vowel in that position. The first-syllable stress holds in both varieties.
I'll probably be late.
How your mouth makes the word
Start PROB with slightly rounded lips and the tongue low and back for the /ɒ/ vowel, then close your lips fully for the /b/.
Let the middle syllable collapse into a quick, relaxed uh; keep it short and unstressed so the beat stays on the first syllable.
Finish blee: curl the tongue tip up to touch the ridge behind your top teeth for the l, then release into a short, quick i sound, like the vowel in bit.
Say it in a phrase
I'll probably stay home tonight. Three syllables with primary stress on the first one. This transcription uses the British-style /ɒ/ vowel, the short rounded sound you hear in hot and stop. Speakers using an American-style transcription would write the vowel as /ɑ/, as in father.
That's probably true. In relaxed speech many speakers drop the middle syllable, so it sounds closer to PROB-lee, two beats with the first one strongest.
Probably has two common shapes. The full form is /ˈprɒbəbli/, three beats with the strongest one on PROB at the start. In relaxed speech many speakers drop the middle syllable, so That's probably true becomes closer to PROB-lee, two beats with the first one strongest.
The ending of probably is light and unstressed, an uh-bl sound followed by a final short i, like the vowel at the end of happy. The last syllable never takes stress; the beat stays firmly on PROB.