From a (as a negative particle) and a presumed derivative of a compound of ek and a derivative of ichnos; not tracked out, i.e. (by implication) untraceable -- past finding out; unsearchable.
KJV: Unto me, who am less than the least of all saints, is this grace given, that I should preach among the Gentiles the unsearchable riches of Christ;
NASB: To me, the very least of all saints, this grace was given, to preach to the Gentiles the unfathomable riches of Christ,