Either inherited from Proto-Germanic *jehwlą or its plural *jeulō, or else borrowed from the Norse cognate jól, from the Proto-Germanic plural.
ġēol or ġeōl n
There is some uncertainty as to how this word was pronounced. The Anglo-Saxon alphabet normally maintained a close match between sounds and letters, but the sequence ⟨geo⟩ was an exception. It could represent /jo(:)/, with the ⟨e⟩ serving only to mark a palatal ⟨g⟩; thus ġeoc (“yoke”) was /jok/ and ġeōmor (“miserable”) was /ˈjo:.mor/. In other cases, it signified a genuine diphthong, as in ġeolu /ˈje͜o.lu/ (“yellow”), ġeorn /je͜orn/ (“eager”), and ġeorran /ˈje͜or.rɑn/ (“to chatter”). It was even the most frequent method of rendering /ju(:)/ in non-Latinate words, since for unknown reasons the Anglo-Saxons did not spell this with ⟨geu⟩: ġeō /ju:/ (“long ago”), ġeong (“young”).[1] Incredibly, geol appears to have been pronounced all three ways: /je͜oːl/, the inherited pronunciation; /jo:l/, from Old West Norse; and /ju:l/, from Old East Norse.
Some authorities rule out /je͜oːl/, since there is no trace of it in Middle English, and /jo:l/ and /ju:l/ are confirmed by the alternative spellings ⟨iol⟩ and ⟨iul⟩. However, there are good reasons to believe the native form existed simultaneously. Norse loanwords are almost all non-West Saxon and late, but geol appears already in Alfred's West Saxon laws c. 893, and in the Old English Martyrology, which was composed sometime in the 9th century though in Mercia. The similar term ġēola (“November and December”) looks to be derived from ġēol and appears very early in Bede's 8th century Ecclesiastical History (c. 731), written well before the first Viking raids. Furthermore, there is a common variant ġeohhol which could only be from Proto-West Germanic *jehwl, with irregular doubling of a velar before /w/. /je͜oːl/ is exactly the expected outcome of *jehwl by regular sound change, and in fact the same development produced the identically-shaped doublets hwēol and hweohhol from PWGmc *hwehwl.
Finally, it is not surprising that /je͜oːl/ would show no trace in Middle English, since no non-northern form of the word does, that is no form but West Norse /jo:l/. This is true even though in Old English, spellings that point to /jo:l/ are rare (e.g., ⟨iul⟩ is much more frequent than ⟨iol⟩).
What appears to have happened is this. The native form /je͜oːl/ existed at least before the first Viking settlements in the 9th century. Beyond this, it could have easily survived in Wessex, where the Danelaw did not rule and Norse loanwords were very rare. Then in late Old English, geol was replaced by Cristesmæsse (first attestation: 1038), except in some areas with Norwegian settlers where West Norse /jo:l/ persisted, wiping out all other forms. According to this explanation, the spelling ⟨iol⟩ was rare only because most West Norse speakers settled in northern areas far away from Wessex, the center of scribal production in Anglo-Saxon England.