Th-stopping

Th-stopping is the realization of the dental fricatives [θ, ð] as stops—either dental or alveolar—which occurs in several dialects of English. In some accents, such as of Indian English and middle- or upper-class Irish English, they are realized as the dental stops [t̪, d̪] and as such do not merge with the alveolar stops /t, d/; thus, for example, tin ([tʰɪn] in Ireland and [ʈɪn] in India) is not a homophone of thin [t̪ʰɪn]. In other accents, such as varieties of Caribbean English, Nigerian English, Liberian English, and older, rural, or working-class Irish English, such pairs are indeed merged. Variation between both dental and alveolar forms exists in much of the working-class English speech of North America and sometimes southern England. It is also common for babies and toddlers, who are still learning to talk and/or haven't fully grown their front teeth capable of producing the th sound. Th-stopping occurred in all continental Germanic languages, resulting in cognates such as German die for "the" and Bruder for "brother".
For the working class of New York City and its surrounding region, the fricatives /θ/ and /ð/ are often pronounced as affricatives or stops, rather than as fricatives. Usually they remain dental, so that the oppositions /t-θ/ and [d-ð] are not lost. Thus thanks may be pronounced [θæŋks], [tθæŋks], or [t̪æŋks] in decreasing order of occurrence; all are distinct from tanks [tʰæŋks]. The [t̪] variant has a weakish articulation. The /t-θ/ opposition may be lost, exceptionally in the environment of a following /r/ (making three homophonous with tree), and in the case of the word with, (so that with a may rhyme with the non-rhotic pronunciation of bitter-bidder; with you may be [wɪtʃu], following the same yod-coalescence rule as hit you. These pronunciations are all stigmatized.
The [d-ð] opposition seems to be lost more readily, though not as readily as the "Brooklynese" stereotype might lead one to believe. As in many other places, initial [ð] is subject to assimilation or deletion in a range of environments in relatively informal and/or popular speech, e.g. who's there [huz (z)ɛə]; as in many other places, it is also subject to stopping there /dɛə/. This option extends to one or two words in which the /ð/ is not initial, e.g. other, which can thus become a homonym of utter-udder. But it would not be usual for southern to be pronounced identically with sudden or breathe with breed.
In African-American Vernacular English, in the words with and nothing, [t] may occur corresponding to standard [θ], with the [t] itself being succeeded by the t-glottalization rule: thus [wɪʔ] for with and [ˈnʌʔɪn] for nothing. Th-stopping is also reported for some other non-initial [θ]s, apparently particularly when preceded by a nasal and followed by a plosive, as keep your mouth closed. In initial position, [θ] occurs in AAVE just as in standard accents: thin is [θɪn], without the stopping of West Indian accents. Stopping of initial [ð], however, is frequent, making then pronounced as [d̪ɪ̃n] or sometimes homophonous with den.
Th-stopping is also commonly heard, specifically from speakers of working-class origins, in the American English dialects of the Inland North (for example, in Milwaukee, Chicago, Detroit, Cleveland, and Buffalo), the Upper Midwest (for example, in the especially Fennoscandian-descended locals of Minnesota's Iron Range and Michigan's Upper Peninsula), and the Mid-Atlantic region (for example, in Philadelphia and Baltimore). It is also heard in a minority of speakers of England's Estuary dialect (for example, in London), but only in the case of word-initial /ð/. Many speakers of Philippine English and some speakers of other variants in Asia also have th-stopping.
The dialect of Sheffield in England is sometimes referred to as "dee-dar" because of the th-stopping to change initial /ð/ to /d/. However, a 1997 study in Sheffield found this was then largely confined to older males.
In certain international radiotelecommunications such as the NATO phonetic alphabet, the digit code words three and thousand are pronounced [tʰɹiː] and [ˈtʰaʊz(ə)nd] to avoid mispronouncing them as [sɹiː] and [ˈsaʊz(ə)nd].