The first element gleann ‘glen’ is not in doubt; the second or specific element however may have originally been from eagas ‘notch-place’ a derivative of eag ‘notch’. This was apparently not readily understandable in Gaelic however and it seems to have been interpreted later as èagas ‘bard or learned man’.
It is not until the seventeenth century that forms with -l- appear, for example Gleneglis in 1664. The name appears to have been reanalysed yet again as Gleann na h-Eaglais ‘the glen of the church’, possibly by non-local Gaelic speakers who used this glen for droving cattle. Finally, within a Scots context, the form eaglais was reinterpreted as if it were the English word eagles, leaving us with the modern form.
For more information on this name visit Ainmean-Àite na h-Alba