When you say "上当", all the Chinese know which two words they are, so don't worry about it. This word stems from an old story happened in Qing Dynasty. Well, I have no time to translate it. I'm copying the story as follows. You can read it if you are interested. 清朝末年,清河有一个大户人家姓王,世代经营当铺,家大业大,生意兴隆。生活富裕了,各房的族人开始懒于经营,把资金存入当铺做入股的股东,日常的典当营业事务全交给一个名叫寿苎的年轻人来主持。寿苎酷爱读书,喜欢校刻书籍,对生意既无兴趣更不精通,处理典当业务非常随便。王氏族人见此情景,都认为有机可乘,不约而同地从自己家中拿一些无用的东西到当铺来典当。各人估定了高于物品本身的价格,要伙计如数付给,伙计不敢得罪股东老板,寿苎也心不在焉,不加阻拦。 　　就这样,没过两个月,典当的资本就被诈骗得差不多了,一家资金充足的当铺破产了。因此,当时流传着这么一首民谣：清河王,自上当,当得当铺空了档。 　　由此可见，“上当”原指去当铺典当东西,后来人们就把受骗叫“上当”。

like when somebody sets you up you can say 我上你的当了。 Always followed by the C word ( F*** in English ) ;)

be careful when using this structure. many times i screwed up the word order and its meaning was opposite. 我上你的当 vs. 你上我的当

就是这个上当

上当 is a set phrase you can never confuse it when it's used in the sentence