How did the j get the dʒ sound?

Question

The j getting the dʒ is very weird, how did the letter j get the dʒ sound?

Why not a /j/ sound as in "yes"?

Answer 1

The letter <j> originated as a variant of the letter <i>, and only came to be viewed as a separate letter relatively recently.

So we need to actually go back to the pronunciation of the letter <i>. In Latin, this letter could have two pronunciations, as a vowel /i/ (either short or long), or as a glide /j/.

In Italo-Western Romance, this phoneme fortified /ʒ/ (the exact outcome varies by language, often merging with the outcome of Latin /g/ before a front vowel, with Spanish going further, devoicing and backing it to /x/).

It's at this point that the Norman conquest occurs, bringing with a large number of Northern Old French loanwords, which English generally preserved the spelling of.

Then the printing press comes along, and someone realises that they've got two variant shapes for the letter <i>: <i> & <j>, and this single letter also has two very different pronunciations and decides that it would be a lot simpler if they always wrote one sound with on shape, and the other with the other. This is when <j> becomes a letter in its own right, and it catches on spreads rapidly across Europe, coming to England.

English has retained the Northern Old French reflex of consonantal <i> /dʒ/, and so, when we adopt <j> as an independent letter, that becomes its sound. The same sound is usually spelt <dge> in native words, and is also spelt <ge> in many French loans, especially where it results from a Latin /k/ or /g/.

Meanwhile, in the Romance languages, the letter <j> takes on whatever sound is the reflex of consonantal <i> e.g. /ʒ/ in French, /x/ in Spanish, etc. To the North and East in Germanic, Slavic, and Uralic, they did not have such a large proportion of Romance loans and instead used this new letter to spell their own consonantal <i> and so in those languages it was used to spell /j/, a sound it retains to this day.

To illustrate, let's look at the history of the word justice:

• Latin: <iústitia> /juːsˈti.ti.a/ (note that, contrary to common statements, Latin did generally mark long vowels, but with an apex, appearing roughly the same as a modern acute accent, rather than the macron more commonly used today). Evolves into:
• Old Northern French /dʒusˈtisə/ is borrowed as:
• Middle English /dʒusˈtis(ə)/
• Slightly Later French /ʒusˈtisə/
• J then becomes an independent letter, and the spelling shifts leaving the pronunciation in place.
• Middle English <justice> /dʒusˈtis(ə)/
• Slightly Later French <justice> /ʒusˈtisə/
• A few centuries more of evolution:
• Modern English <justice> /ˈdʒʌstɪs/
• Modern French <justice> /ʒys.tis/

In parallel, we also have

• German <Justiz> /jʊsˈtiːts/
• Spanish <justicia> /xusˈtiθja/

And so the English speaker sees the letter <j> and thinks /dʒ/, the French speakers sees the letter <j> and thinks /ʒ/, the Spanish speaker thinks /x/, and the German thinks /j/.

Answer 2

The sound comes first, the writing is fit around the sound. The sound [dʒ] in Modern English is a late-comer, and was not significant in Old English. Old French [dʒ], which was more significant, derived from various (post-)Latin sources, including i~j (also *g plus front vowel). The English practice of writing <j> for [dʒ] was borrowed from French (as were words so spelled). Other Germanic (and neighboring Uralic) languages were not faced with a large set of words containing [dʒ] which had to be accommodated in the writing system, hence German, Dutch and Norwegian j is pronounced as a glide. J being a later graphemic adaptation of i, it was not strongly bound to an antecedent pronunciation from Latin.