The Free On-line Dictionary of Computing (19 January 2023):

loose bytes

   Commonwealth hackish term for the padding bytes or shims
   many compilers insert between members of a record or
   structure to cope with alignment requirements imposed by the
   machine architecture.


The Jargon File (version 4.4.7, 29 Dec 2003):

loose bytes
 n.

    Commonwealth hackish term for the padding bytes or shims many compilers
    insert between members of a record or structure to cope with alignment
    requirements imposed by the machine architecture.