This is the 6811th most frequent Japanese word.
フニャフニャ
Soft, limp, or pliable; lacking firmness or structure.
In this sentence, 'フニャフニャ' is used to describe the way of speaking which sounds vague or lacking firmness.
彼の話し方はいつもフニャフニャしている。
His manner of speaking is always soft and unclear.
Here, 'フニャフニャ' characterizes the physical condition of the cushion being soft and squishy.
古い椅子のクッションがフニャフニャになってしまった。
The cushion of the old chair has become squishy.
In this example, 'フニャフニャ' conveys a metaphorical weakening or loss of strength in confidence.