inexpressiblein‧ex‧pres‧si‧ble /ˌɪnɪkˈspresəbəl◂/ adjective formal 
- And then, to her almost inexpressible joy, she saw the familiar tall, broad-shouldered figure across the station.
- Looking up into his concerned brown eyes, I had felt an inexpressible sense of relief.
when it is difficult to express ideas or feelings in words► can't express · She couldn't express how happy and relieved she was.· I cannot express my gratitude for your kindness.words can't express · Words can't express the terrible grief I felt on losing my daughter.
► be difficult/hard to describe to be difficult to express what a feeling, experience, taste etc is like in words: · The atmosphere of tension and nervousness here is hard to describe.· The sound the bird makes is difficult to describe -- it's a bit like scissors opening and closing rapidly.
► indescribable if emotions, situations etc are indescribable , they are so terrible, so good, or so strange that you cannot describe them properly: · The chaos and confusion was indescribable.indescribable joy/peace/horror/sadness etc: · I woke up on the day after my wedding with a feeling of indescribable joy.
► inexpressible formal an inexpressible emotion is one that is too strong to be described in words: · Looking up into his concerned brown eyes, I had felt an inexpressible sense of relief.inexpressible anger/gratitude/joy/longing etc: · And then, to her almost inexpressible joy, she saw the familiar tall, broad-shouldered figure across the station.
adjectiveexpressive ≠ inexpressiveexpressionlessinexpressibleadverbexpressivelyexpressionlesslyinexpressiblynounexpressionexpressivenessverbexpress
inexpressible gratitude—inexpressibly adverb