1813, name for various kinds of liquor drinks, or for intoxicating drinks generally, possibly a variant of switchel "a drink of molasses and water" (often mixed with rum), first attested 1790, of uncertain origin. Swizzle-stick attested by 1859.
To convert external names, array indices, or references within a data structure into address pointers when the data structure is brought into main memory from external storage (also called "pointer swizzling"); this may be done for speed in chasing references or to simplify code (e.g. by turning lots of name lookups into pointer dereferences). The converse operation is sometimes termed "unswizzling".
See also snap.