whistle 既是名词“口哨”，又是动词“吹口哨” ”哨子“ 用金属或塑料等制成的能吹响的器物，多在集合人员、操练或体育运动时使用。 “口哨” 指用口的作用通过双唇、舌头和气流，就能发出“萧”一样的声音。 “哨” 1.侦察；巡逻 2. 为警戒、侦察等任务而设的岗位 相关“哨”的词组有： 哨子，口哨，哨兵，暗哨，岗哨，查哨 等等。

I would say the the direct mapping word in Chinese is 吹口哨 for whistle, as a verb, 口哨 or 哨子 as a noun. It is not that logical to understand a single character of Chinese with a word or context. However, if you insist on meaning of the character 哨 itself, it should be the same as 口哨 or 哨子 in general - the metal or plastic or whatever object with a tube, which you blow in order to produce a sound to attract attention. However, there is other words or context where the 哨 is seen, for example, 放哨 or 哨兵 (meaning soldiers, police, or guards patrol an area or building).