WordNet sense 1 (too sacred to be uttered):

ineffable, unnameable, unutterable

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

sacred

WordNet sense 3 (exceptionally bad or displeasing):

abominable, atrocious, awful, dreadful, painful, terrible

WordNet sense 4 (having undesirable or negative qualities):

bad

WordNet sense 5 (defying expression or description):

indefinable, indescribable, untellable

WordNet sense 6 (defying expression):

inexpressible, unexpressible