Why does “a pesar de” signify “in spite of”?

Question

In Spanish and Portuguese, a pesar de means in (de)spite of. But in both languages, pesar has never meant spite – see quotation below on the etymology of spite. pesar hails

From Old Spanish pesar, from Vulgar Latin *pēsāre, from Latin pēnsāre, present active indicative of pēnsō.

How does a pesar de yield the meaning in spite of?

spite [13]

Spite was adapted from Old French despit ‘scorn, ill will’, which was also borrowed intact as despite [13]. This came from Latin dēspectus, the past participle of dēspicere ‘look down on’ (source of English despise [13]), which was a compound verb formed from the prefix dē- ‘down’ and specere ‘look’ (source of English spectacle, spy, etc). The use of in spite of and despite for ‘notwithstanding’ goes back via an intermediate ‘in defiance of’ to an original ‘in contempt of’.

Word Origins (2005 2e) by John Ayto. p 473 Left column.

Answer 1

According to this paper (see pages 8 to 11), "pesar" in the lexicalized expression "a pesar de" is related to the meaning of "pesar" as "regret" or "sorrow" and was initially used only for people ("a pesar de alguien": to the regret of somebody):

(13) Ellos las han dexadas a pesar de nós” (Cid) (Ellos las han abandonado para nuestro pesar: They have abandoned them to our regret)

Gradually, the expression started to be used with any kind of grammatical object, for example:

• Ellos las han abandonado a pesar de nuestra súplica (they have abandoned them in spite of our request).

I think some kind of parallel can be drawn with the meaning of the noun "spite" meaning "anger". "In spite of" would then stem from an expression meaning that something caused somebody's anger, something similar to "showing contempt or scorn for" (She married him in spite of me) and gradually evolved to refer to animate and inanimate objects (She married him in spite of my warning).

Answer 2

Pesar also means "to weigh" in Spanish, so it can also be viewed as signifying an occurrence in spite of the burden that something else caused, which could have impeded it.