As a new type of blackhat Search Engine Optimization (SEO), autocomplete manipulations are increasingly utilized by miscreants and promotion companies alike to advertise desired suggestion terms when related trigger terms are entered by the user into a search engine. Like other illicit SEO, such activities game the search engine, mislead the querier, and in some cases, spread harmful content. However, little has been done to understand this new threat, in terms of its scope, impact and techniques, not to mention any serious effort to detect such manipulated terms on a large scale. Systematic analysis of autocomplete manipulation is challenging, due to the scale of the problem (tens or even hundreds of millions suggestion terms and their search results) and the heavy burdens it puts on the search engines. In this paper, we report the first technique that addresses these challenges, making a step toward better understanding and ultimately eliminating this new threat. Our technique, called Sacabuche, takes a semantics-based, two-step approach to minimize its performance impact: it utilizes Natural Language Processing (NLP) to analyze a large number of trigger and suggestion combinations, without querying search engines, to filter out the vast majority of legitimate suggestion terms; only a small set of suspicious suggestions are run against the search engines to get query results for identifying truly abused terms. This approach achieves a 96.23% precision and 95.63% recall, and its scalability enables us to perform a measurement study on 114 millions of suggestion terms, an unprecedented scale for this type of studies. The findings of the study bring to light the magnitude of the threat (0.48% Google suggestion terms we collected manipulated), and its significant security implications never reported before (e.g., exceedingly long lifetime of campaigns, sophisticated techniques and channels for spreading malware and phishing content).