From the OED.  It does not give an etymology, however:

 

Sigaldry:

Obs. rare.

 

    Enchantment, sorcery.
 
  a1225 [see SIGALDER n.]. 13.. K. Alis. 7015 (W.), Quede and harme he wil me spye,..Gef he wot of this sygaldrye [Laud trigoldrye] That this trowes [= trees] kan lye. ?a1500 Chester Pl., Crucifixion (Shaks. Soc.) II. 69 Burye hym wher thy wil be, But look thou make no sigaldry To raise him up agayne.

Sigalder:

Obs. rare.

    A charm or incantation.
 
  c1000 Saxon Leechd. I. 388 Sye-ealdor ic begale, sie ~yrd ic me wee . a1225 Ancr. R. 208 Sigaldren [v.r. sigaldrie], & false teolunges, leuunge on ore & of swefnes, & alle wichchecreftes.
 

    Hence sigalder v., to enchant, bewitch. Obs.
 
  1303 R. BRUNNE Handl. Synne 503 ere was a wycche, and made a bagge, A bely of leyr ,..She sygaldryd so ys bagge bely at hyt ede and soke mennys ky.