How I remember being told over and over how to spell tongue! I didn’t understand it then; I don’t understand it now. What evolution might put a silent “ue” at the end of a word?
In Old English, a language from the West Germanic family, the standard spelling was tunge (wiktionary) and the corresponding pronunciation was /ˈtʊnɡe/ “tun-ghe” (/ʊ/ as in foot). In other words, the final “e” was not mute but clearly pronounced. For instance in present-day German, you would say “Die Zunge” /ˈtsʊŋə/ (tsung-e). In both cases the final unstressed e is actually already a reduction of an older final a still present in many other cognates.
The spelling of Middle English is strongly influenced by that of Old French in which the un sequence is uncommon. It was thenceforth often replaced by on which led to such spellings as tonge or tounge (‘ou’ being the French spelling of /u/ and /ʊ/) . Which can be compared to contemporary alternative spellings longe and lounge for the word long.
With the evolution of the pronunciation of long vowels in Modern English, these spellings became highly misleading. “Tounge” for instance could be read /taʊndʒ/ (as in lounge). As a result, such spellings as tonghe or tongue were preferred. The fact that the simpler spelling tong was not preferred (in spite of the existence of such words as long) is probably a sign that the final e was not completely muted yet.
Finally, the tongue variant probably overtook the tonghe option as a result of an analogy with such words as L. lingua or languages.
Also note that on the other side of the English Channel, the Dutch word followed a very similar path. From Old Dutch tunga it successively evolved as Middle Dutch tunge, tonghe and present-day Dutch tong.
This is the explanation offered by the OED regarding the spelling and the etymology of the word tongue.
[OE. and ME. tunge wk. f. = OFris.
tunge, OS. tunga (MLG., LG. tunge,
MDu. tonghe, Du. tong), OHG. zunga,
zunka (MHG., Ger. zunge), ON. tunga
(Da., Norw. tunge, Sw. tunga), Goth.
tuggô:—OTeut. *tungôn-, held to be
cogn. with L. lingua tongue, for older
*dingua (as lacrima:—dacrima: see tear n.1).
The natural mod.Eng. repr.
of OE. tunge would be tung, as in
lung, rung, sung (and as the word is
actually pronounced); but the ME.
device of writing on for un brought in
the alternative tonge with variants
tounge, townge; app. the effort to
show that the pronunciation was not
(tundʒ(ə) led to the later tounghe,
toungue, tongue, although it is true
that these hardly appeared before
final e was becoming mute, so that its
simple omission would have been
equally effective. The spelling tongue
is thus neither etymological nor
phonetic, and is only in a very small