According to the DEAF the word can also be interpreted as an adverbial formation of aate ('suitable, well-formed'). However, the short first vowel suggests that aptus (for apte) is a more probable etymon than adaptus (for aate).
adv.
1appropriately, fittingly:
(
MS: 1120-40
)
Ed attement le pose a la terre: Felix le liu u sun saint cors herberget!
569
(
s.xiiiex; MS: s.xivin
)
Ki se repent parfitement Et puys se tient bien attement Quant il out mesfait ainz ancis Vers Dieu recovre tot son pris
206
[aptement] (1120-40)