As the old English expression goes: "A fool and his money are soon parted." Some people have "deeper pockets" than others. Often, those people with the deep pockets are surrounded by others who sense that heavy wallet and will take advantage of that person's financial generosity.
In English, this person may be called a "big spender", which doesn't sound so bad, or "gullible", which has a more negative meaning. When the surrounding people or entrepreneurs are taking advantage of someone who is in the position to pay the bill or buy a product we have a word to describe the unfortunate "victim" of this greed. This kind of "sucker" could be called "冤大头 (yuān dà tóu)" in Chinese.
Often, the "sucker" gets stuck with the entire bill at a restaurant. Other diners at the table may sense that the "冤大头 (yuān dà tóu)" will be paying the check, so they may order expensive items from the menu. Maybe the "冤大头 (yuān dà tóu)" always ends up "buying too many rounds" at the bar. Perhaps on holiday, a tour guide or resort may expect the "冤大头 (yuān dà tóu)" to pay a lot more for food and services than the locals do, since the "冤大头 (yuān dà tóu)" is new to the country.
So, what exactly is a "冤大头 (yuān dà tóu)"?
Let's deconstruct "冤大头 (yuān dà tóu)":
冤 (yuān) means: bad luck/enmity/injustice/wrong.
大 (dà) is a very common word meaning "big."
头 (tóu) means head.
大头 (dàtóu) is the larger and thicker end of something; the largest share; the major part.
Originally, the phrase "冤大头 (yuān dà tóu)" referred to spending money unnecessarily. Today, it is used to refer to a person who is deceived or taken advantage of on account of his or her generosity. Of course, it can be referred to manipulated and bullied people who have wasted their money. In English, it's not uncommon for this person to be referred to as a "sucker".
Wǒ shénme shíhou dāng yuān dà tóu le?
When was I the sucker?
Jack jīntiān zuò le cì yuān dà tóu.
Jack got ripped-off at the market today.