WordNet sense 1 (too sacred to be uttered):

ineffable, unnameable, unspeakable

WordNet sense 2 (concerned with religion or religious purposes):

sacred

WordNet sense 3 (defying expression or description):

indefinable, indescribable, untellable

WordNet sense 4 (defying expression):

inexpressible, unexpressible

WordNet sense 5 (very difficult to pronounce correctly):

unpronounceable

WordNet sense 6 (not inclined to talk or give information or express opinions):

uncommunicative, incommunicative