Weak Looking
Frail, Weak Seeming, Seemingly Weak
い adjective
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.
弱々しい光
weak light
弱々しい声
feeble voice
弱々しいすがた
weak looking appearance
主人は弱々しい体ながら全力で走った。
My husband ran with all his might, despite his weak-looking body.
その車両は弱々しいエンジン音を立てていた。
The vehicle made a frail engine sound.
母は弱々しい声で、おばあちゃんに「ゆっくり、休んで」と言った。
My mother in a weak voice told her grandmother, "Take time to rest."