Sending flowers to her when she was ill was such a thoughtful gesture, a true thought of kindness.

thought 是名词，来自动词 think（思考），本义是"想法、思考"。 这里 thought 与 of kindness 搭配，表示"关怀、体贴的想法"，强调这种善意是经过深思熟虑的。