The matching adjective, severe, appears to be unattested in Anglo-Norman. A single instance in Lapid 276.291 is not a genuine attestation, but an editorial modification of the manuscript reading serré (cf. serré1).
adv.
1severely, harshly:
(
1325
)people are being drowned on the sands of Morecambe Bay si severement qe pité deust prendre chescun cristien
i 436
severement1 (1325)