The Dwarves may have been secretive, but their language wasn't a completely closed book (unlike their secret names).  In his Essay Of Dwarves and Men in 'The Peoples of Middle-Earth' he says that, at the end of the First Age, Dwarves and Men, particularly those of the House of Hador, became quite close as trading partners and as allies against the Dark forces, and Dwarves "...were not unwilling to teach their own tongue to Men with whome they had special friendship, but Men found it difficult and were slow to learn more than isolated words, many of which they adapted and took into their own language."  I have heard it suggested that the Adunaic number 'seven', given in one place as hazid was derived from the Khuzdul khazad- (the Dwarves own name from themselves) though this is unlikely to be a straight loan, and may refer to the seven fathers of the Dwarves.  Structurally Adunaic and Khuzdul (and Valarin) are close, resembling our Semitic languages such as Hebrew.  The Númenorean verb azgarâ (to wage war) seems very close to the name of Azaghâl, Lord of Belegost, which probable means of warrior.

Some Khuzdul words even made it into some of the Elvish languages. For example the Sindarin word heledh (glass) was derived from the Khuzdul kheled, and the Elves adopted Khazad. In the Second Age Dwawrves taught some Elves their tongue purely out of interest, which they respected.  As Tolkien puts it, "...some of the later Ñoldorin loremasters were allowed to learn enough of both their lambe (aglâb) ["tongue" in Quenya and Khuzdul] and their iglishmêk [gesture-code] to understand their systems."  Elves such as Curufin, the loremaster Pengolodh, and probably even Galadriel (a very Khazadophile Elf if ever there was one) seem to know at least some of the language.  Remember her speach to Gimli, where she remembers for him the glory of Khazad-dûm?