The Collaborative International Dictionary of English v.0.48:

Imprimis \Im*pri"mis\, adv. [L., for in primis among the first,
   chiefly; in in + primus first.]
   In the first place; first in order.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

IMPRIMIS. In the first place; as, imprimis, I direct my just debts to be
paid. See Item.