(Donguri no seikurabe; “A height-measuring contest between acorns”)
A group where none of the members particularly stand out from the rest. A collection of average types. Alternately, a pointless competition between people of no special ability; a pissing contest. Like comparing acorns, which are all about equally small.
This noun phrase begins with the noun 団栗 (donguri), “acorn.” This is followed by 背 (various pronunciations; here sei), “back,” which joins together with 比ぶ (kurabu), “to compare,” acting as a noun in conjunctive form. The associative particle の (no) links the two halves of the whole.
Kurabe can be written 競べ without any change in meaning or pronunciation.
This phrase apparently comes to us from Kaozheng scholar Qian Daxin (銭大昕), in a work called 恒言録 (Héng yán lù, Japanese pronunciation unclear but probably Kougenroku)
(“Kotoshi no yobisenkyo no kouhosha-tachi wa donguri no seikurabe de, chotto ikishouchin shite shimatta.”)
[“The candidates in this year’s primary race are all just average; it’s kind of a let-down.”]