Literally 'completely prepared' — shifted to 'by this time' because what's fully ready is done before expected.
Before the time in question; by now or by then.
A compound of 'all' + 'ready,' literally 'all ready' — completely prepared, hence 'by this time.' From Old English 'eall' (all, completely) + 'rǣde' (ready, prepared), from Proto-Germanic '*raidijaz' (ready, arranged). The semantic shift from 'completely prepared' to 'by now' happened because if something is 'all ready,' it has been completed before the expected time. Cognate with German
'Already' and 'all ready' were once the same phrase — you can still see the seam. The difference today: 'The bags are all ready' means every bag is prepared, while 'The bags are already here' means they arrived before expected. The same fusion happened with 'altogether' (all + together) and 'although' (all + though). English loves