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Phrase(s): Blood is thicker than water.[and] Blood runs thicker than water.
Prov. People who are related have stronger obligations to each other than to people outside the family. • My friends invited me to go camping on Saturday, but I have to go to my cousin’s wedding instead. Blood is thicker than water, after all. • If you ever need help, don’t ask your friends. Come home and ask us, your family. Blood runs thicker than water.
McGrawhill's American Idioms And Phrasal Verbs
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Family ties are closer than other relationships. For example, Nancy will drop everything to help her sister; blood is thicker than water. Alluding to the fact that water evaporates without leaving a mark whereas blood leaves a stain, this proverb was first recorded about 1412.
American Heritage Idioms