Bless
Old English blēdsian, blētsian, based on blōd ‘blood’ (i.e. originally perhaps ‘mark or consecrate with blood’). The meaning was influenced by its being used to translate Latin benedicere ‘to praise, worship’, and later by association with bliss.
wiktionary
From Middle English blessen, from Old English bletsian(“to consecrate (with blood)”), from Proto-West Germanic *blōdisōn(“to sprinkle, mark or hallow with blood”), from Proto-Germanic *blōþą(“blood”), of uncertain origin, possibly from Proto-Indo-European *bʰleh₃-(“to bloom”). Cognate with Old Norse bleza(“to bless”) (whence Icelandic blessa), Old English blēdan(“to bleed”). More at bleed.
An ellipsis for an expression such as bless your heart.
etymonline
bless (v.)
Old English bletsian, bledsian, Northumbrian bloedsian "to consecrate by a religious rite, make holy, give thanks," from Proto-Germanic *blodison "hallow with blood, mark with blood," from *blotham "blood" (see blood (n.)). Originally a blood sprinkling on pagan altars.
This word was chosen in Old English bibles to translate Latin benedicere and Greek eulogein, both of which have a ground sense of "to speak well of, to praise," but were used in Scripture to translate Hebrew brk "to bend (the knee), worship, praise, invoke blessings." L.R. Palmer ("The Latin Language") writes, "There is nothing surprising in the semantic development of a word denoting originally a special ritual act into the more generalized meanings to 'sacrifice,' 'worship,' 'bless,' " and he compares Latin immolare (see immolate).
The meaning shifted in late Old English toward "pronounce or make happy, prosperous, or fortunate" by resemblance to unrelated bliss. Meaning "invoke or pronounce God's blessing upon" is from early 14c. No cognates in other languages. Related: Blessed; blessing.
