unpaid minimum required contribution
(A)The term “unpaid minimum required contribution” means, with respect to any plan year, any minimum required contribution underfor the plan year which is not paid on or before the due date (as determined under section 430(j)(1)) for the plan year. (B)Any payment to or under a plan for any plan year shall be allocated first to unpaid minimum required contributions for all preceding plan years on a first-in, first-out basis and then to the minimum required contribution underfor the plan year.