What does "FG BNK" mean in a metar?
It means fog bank from S to NW of the station, although it's not codified.
The remarks section (after RMK) does not always use official codes. In this case, BNK neither appears in the FAA's 7900.5, nor WMO publications.
To quote the 7900.5:

14.18. Remarks (RMK)
(...)
Where plain language is called for, authorized contractions, abbreviations, and symbols should be used to conserve time and space. However, in no case should an essential remark, of which the observer is aware, be omitted for the lack of readily available contractions. In such cases, the only requirement is that the remark be clear. For a detailed list of authorized contractions, see FAA Order 7340.1, Contractions.

By checking 7340.1, BNK doesn't appear on its own or with FG, only in GRBNKS and SNBNK. Since no contraction in 7340.1 was available, the manual entry was made in as clear manner as possible.
For completeness, if the fog bank had affected the station, the proper code that would have been used before RMK would have been BCFG (fog patches). BC is likely derived from the French banc de brouillard (fog bank).