Dictionary of Biblical Words:
This means one who is taking the responsibility due to another, as in endorsing a bill of exchange, &c. Christ is thus the sinner’s surety, taking upon Himself all the responsibility of his sine, and answering to God for them all. Humanly speaking suretyship is condemned (see Prov. 11:15), but without a Divine surety we all were lost.
Strong’s Dictionary of Greek Words:
Meaning:
from 1722 and γυῖον (a limb); pledged (as if articulated by a member), i.e. a bondsman