A mentada de madre (lit. mention of one's mother), or just mentada, is possibly the top insult in Mexico. It's usually expressed as ¡chinga tu madre! or ¡vas y chingas a tu madre! It means "fuck your mother" but can be interpreted as "go fuck yourself" too. A sole mentada is enough to get you in a lot of trouble. Unfortunately, the rush of modern life gives us less time than we would like to have to use all the palette of swearing. Hence people, especially motorists and bike-riders, have popularized a shorter, single-noted musicalized version of chinga tu madre: the whistle- or horn-mentada. Few words or sounds have the strength of a well deserved horn-mentada on a reckless driver.
Listen to it, feel it and learn it by heart. Timing is the key element. Nothing can be more embarrasing on the streets of Mexico City than an arythmic mentada.
For the cases in which a mentada cannot or must not be heard, there is a silent option. All that is needed is to flex the forearm up, from a horizontal to a vertical position forming an L (as an arm curl). It is the weakest version and is often used when insulting someone (typically a classmate or a coworker) across the room without the rest noticing. Whenever possible go for a deep, loud ¡chinga tu madre! instead.