Literally 'all most' — promising completeness then retracting it, a paradox unique among Germanic languages.
Not quite; very nearly; all but.
From Old English 'ealmǣst,' a compound of 'eall' (all, wholly) + 'mǣst' (most, greatest). The literal meaning is 'all most' — i.e., 'for the most part wholly, nearly entirely.' This is the same 'al-' intensifier seen in 'alone' (all one), 'always' (all ways), 'already' (all ready), and 'altogether' (all together). The word has meant 'nearly' since Old English, showing that the semantic shift from 'mostly all' to 'not quite all' occurred very early. Key roots: eall (Old English: "all, whole, entire"), mǣst (Old English: "most, greatest, largest").
'Almost' is 'all + most' — nearly all, for the most part. But notice the subtle paradox: 'almost' means 'not quite,' so 'all most' actually means 'not all.' The word promises completeness ('all') and then withdraws it with the qualifier ('most' = the greater part but not the whole). Every other Germanic language uses a completely different construction: German