I was starting to worry about you because you had such a weak singing voice.
My father, who used to look strong, gradually began looking weaker year after year.
That weak looking patient is suffering from puffer fish toxins.
Something that is double weak is probably pretty weak. You also know that this is an adjective because it ends with い. You know that 弱い is weak, so it probably isn't that. Instead, this one is weak looking or frail. If you know the grammar らしい you can kind of use that as a hint. The しい on the end reminds me of らしい which is like "seems to be." So, "seems to be weak" = "weak looking."
The reading is double kun'yomi, as you can probably guess due to all the hiragana involved. So, you double up on よわ and then add the しい to read this vocabulary word.