Wikipedia
In English phonology, t-glottalization or t-glottaling is a sound change in certain English dialects and accents that causes the phoneme to be pronounced as the glottal stop in certain positions. It is never universal, especially in careful speech, and it most often alternates with other allophones of such as , , (before a nasal), (before a lateral), or .
As a sound change, it is a subtype of debuccalization. The pronunciation that it results in is called glottalization. Apparently, glottal reinforcement, which is quite common in English, is a stage preceding full replacement of the stop, and indeed, reinforcement and replacement can be in free variation.
The earliest mentions of the process are in Scotland during the 19th century, when Henry Sweet commented on the phenomenon. The SED fieldworker Peter Wright found it in areas of Lancashire and said, "It is considered a lazy habit, but may have been in some dialects for hundreds of years." David Crystal claims that the sound can be heard in Received Pronunciation (RP) speakers from the early 20th century such as Daniel Jones, Bertrand Russell and Ellen Terry. The Cambridge English Pronouncing Dictionary claims that t-glottalization is now most common in London, Leeds, Edinburgh, and Glasgow.