From Wikipedia:
Mamihlapinatapai (sometimes spelled mamihlapinatapei) is a word from the Yaghan language of Tierra del Fuego, listed in The Guinness Book of World Records as the „most succinct word“, and is considered one of the hardest words to translate. It describes a look shared by two people with each wishing that the other will initiate something that both desire but which neither one wants to start. This could perhaps be translated more succinctly as „eye-contact implying ‚after you…'“. A more literal approximation is „ending up mutually at a loss as to what to do about each other“.