stramenticios

I'm a little confused by this word from Harrius Potter. I know it means made of straw. it is the -iciōs (presumably -icius in nom m s) I can't seem to find a reference to this suffix. I am aware of -icus is -icius just an alternative form?
 

Agrippa

Civis Illustris
Addendum:
vespere illius diei, Dudley habitum novissimum familiae in
sessorio ostendit, discipuli scholae Smeltings vestes caudatas
colore purpureo gerebant, bracas solutas transatlantici generis
luteo colore, petasos planos et stramenticios nomine lintrarios
 
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