Why is there a ‘b’ in subtle? And, for that matter, why is there a ‘b’ in debt, doubt or plumber? The letter ‘b’ is not the only seemingly redundant silent letter in English – why is there a ‘p’ in receipt, a ‘c’ in indict or a ‘s’ in isle or aisle.
A sensible guess might suggest that the pronunciation has shifted over the years, leaving the silent letter exposed like a rocky outcrop revealed by a receding tide. Or perhaps it reflects the etymological origins of the word and was retained regardless of its later redundancy.
Few would guess that the redundant and silent letters were later additions, or that their insertion was as a result of intellectual snobbery on the part of middle age and Renaissance scholars.
All of the words listed above entered English from Old French. I will use the example of debt, but the derivation for the others is set out at the end of the post. Debt comes to English from the Old French dette or dete. In both Old French and Middle English, the word had no letter ‘b’ and is completely absent from the pronunciation in both languages.
So where did it come from? Scholars from the middle ages and into the Renaissance started to fully immerse themselves in classical texts and languages. They worked out the ultimate Latin roots of many words, and wanted the Latin original to be made obvious in their ‘modern’ spellings.
Thus the Latin debitum (think of debit) gave its ‘b’ to debt. This happened in both English and French, but Modern French purged itself of redundant letters. The English language, lacking so effective an exfoliant as the Académie française, never lost these letters. As a result they remain, petrified remnants demonstrating the strange and twisting history of language development.
All word references from the Oxford English Dictionary, third edition (September 2006); online version (December 2011)
As a postscript to the above, plumber arrives from the Latin root plumbum, meaning lead. A plumbārius in classical Latin is a worker in lead, and Roman pipes were invariably fashioned from lead. Nowadays, regardless of whether the pipes are lead, iron, copper or plastic, they are all attended to by plumbers.